Optical transmission systems, which are based on wavelength division multiplexing (WDM), achieve high information capacities by aggregating many optical channels, each carrying a specific wavelength band, onto a signal strand of optical fiber. A tunable filter is a critical optical element with several crucial roles in WDM communication systems. A tunable filter, which can redirect and route wavelengths, is used in conjunction with tunable lasers to create a tunable transmitter, midway in the fiber, in wavelength add and drop applications and at the receiving end in conjunction with a broad band detector for a tunable receiver.
It is also known that an optical signal traveling through an optical fiber is subject to variations in its polarization state due to the birefringence of conventional single mode optical fibers. Therefore, an optical signal of a given wavelength λ0 with single polarization state, after traveling some distance through a fiber, will have two signal components of two orthogonal polarization modes (directions), respectively, for example (but not limited to) TE and TM polarization modes, i.e., λ0TE and λ0TM. An optical filter must be able to filter the optical signal irrespective of its polarization state. Hence, it is of utmost importance to provide polarization independent tunable filters, which meet the following criteria:
Common central wavelength λ0, namely, the central wavelengths of both polarizations must be equal (i.e. λ0TE=λ0TM;
The filter shape for both modes must be equal, i.e., g(λ)TE=g(λ)TM;
The filter loss for both polarization states must be identical i.e., LTE=LTM.
The following two solutions for the above problem are known in the art:                1. Careful control of the layer structure and refractive index of a filter device, which provides for a polarization insensitive device (K. Worhoff et al, “Birefringence Compensation Applying Double-Core Waveguiding Structures”, IEEE Photon. Tech. Lett., vol. 11, pp. 206–208, (1999));        2. Polarization diversity, i.e., splitting the incoming signal into its constituent polarization modes and independently filtering each polarization separately.        
FIG. 1 illustrates a prior art device (WO 01/22139) utilizing a polarization diversity scheme. This scheme is aimed at managing the polarization dependence of a microring resonator which employs two resonators serially arranged and constructed to separately filter transverse electric (TE) and transverse magnetic (TM) polarization components of a predetermined wavelength in a randomly polarized DWDM optical signal, and to recombine the separately filtered components prior to output from an optical component employing the inventive polarization diversity scheme. Here, the polarization splitting and subsequent tunable filtering are achieved by using optimized filters for the TE and TM signal.